First, as for the mid-level doldrums, the GM has to learn to challenge the characters, and I’ve discovered that it’s not always easy for DMs to do this, especially at mid levels. (Also, you don’t have to use the Gygax-inspired geometric progression. Rolemaster used a linear progression: 10,000 XP for 1st-10th levels, 20,000 xp for 11th-20th levels, etc… (I could have the exact numbers wrong, it’s been years.))
Secondly, I think a lot of xp systems get bogged down in the false precision of requiring thousands of XP to go up a level. When was the last time you earned 1XP, or cared about earning 1 XP? Even at 1st level in old-school AD&D, that was four hundredths of a percent of the experience award needed to advance to second level thief.
And the problem develops a life of its own as the paperwork required to keep track of all that false precision becomes part of play. My current gaming group has a “Experience Guideline” sheet that one DM made popular, and now no matter who DMs, we are expected to abide by it. It has entries like “Single Kill - XP value of creature” under Fighter and “Spell Cast In Combat - Spell Level times Caster Level times 100.” And players keep long lists of every creature killed and every spell cast, and hand them to the DM at the end of each session so that the DM doesn’t forget to give them their 35 points for a kobold or their 100 points for a cantrip.
It always kind of irritated me. I would boycott note-taking for experience purposes and just give the DM a note at experience-calculating-time saying “I spent my time roleplaying, not min-maxing my notes. You know what I contributed.”
One thing that I wanted to do but was never popular was to use a Character Point system for AD&D 2E with Skills and Powers. Instead of awarding experience, I would just go ahead and award character points according to how you performed with respect to your level of ability. These could be used to buy skills and there would be a simple table for level progression: every 4 character points you earn bumps you up a level.
If you’re first level, cleaning out a Kobold lair may earn you 1 or 2 character points. But doing the same at 10th level wouldn’t earn you squat… there’s a rogue mage/thief leading a band of highwaymen and making local commerce impossible. Neutralize them and you’ll get a couple CPs.
See, that way you adjust the conditions for award instead of the thresholds for gaining a level, and by only counting in single digits, you eliminate all the XP accounting. (Not to mention the possibility of cheating… I encountered one player that would regularly pad his xp awards… forcing the DM to keep track of everyone’s XP total as well… what a pain.)